This invention relates to fuel cell systems.
A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the gases (the anode gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas (the cathode gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode gas flows through the channels of the anode flow field plate, the anode gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode gas flows through the channels of the cathode flow field plate, the cathode gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode gas to catalyze the conversion of the anode gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode gas and the reaction intermediates to catalyze the conversion of the cathode gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Electrons are formed at the anode side of the membrane electrode assembly, indicating that the anode gas undergoes oxidation during the fuel cell reaction. Electrons are consumed at the cathode side of the membrane electrode assembly, indicating that the cathode gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3.
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1) 
xc2xdO2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2) 
H2+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(3) 
As shown in equation 1, the hydrogen is reacted to form protons (H+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water, or other non conducting fluid) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
FIG. 1 shows a fuel cell system 300 including a fuel cell stack 302 having a plurality of fuel cells 304. Fuel cell system 300 also includes an anode gas supply 306, an anode gas inlet line 308, an anode gas outlet line 310, a cathode gas supply 312, a cathode gas inlet line 314, a cathode gas outlet line 316, a coolant inlet line 318, and a coolant outlet line 320.
To increase the electrical energy available, the plurality of fuel cells 304 can be arranged in series, to form fuel cell stack 302. For example, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate, e.g., a monopolar plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
Fuel cell stack 302 is typically provided with inlets, outlets, and manifolds for directing the flow of reactants and coolant to the appropriate flow plates, and assembled between a pair of thick rigid end plates, which are also provided with inlets and outlets. The end plate that is used to deliver one or more reactants to an end of the fuel cell stack is sometimes called a service end plate 321 (FIG. 2). The end plate at the opposite end of the fuel cell stack is sometimes called a blind end plate 323 (FIG. 2). The edges of the two end plates are bolted together to apply a compressive force on the fuel cell stack.
FIG. 2 is a partial schematic representation of fuel cell system 300 in operation. Anode gas supply 306, e.g., a reformer, provides in parallel hydrogen gas via inlet line 308 to the anodes of cells 1 through N, e.g., 88. At each cell, the anode converts the hydrogen into protons and electrons. The protons travel through the solid electrolyte and to the cathode of the respective cells. At cell 1, the electrons flow toward an external load. At the other cells, the electrons flow to the cathode of an adjacent fuel cell, toward the external load. Unreacted anode gas flows through the cells of fuel cell stack 302 through outlet 310.
Similarly, cathode gas supply 312, e.g., an air blower, provides in parallel oxygen (air) via inlet line 314 to the cathode of cells 1 through N. At each cell, the cathode forms water from the oxygen, protons from the respective anode, and electrons flowing from the external load (cell N) or adjacent anode (cell 1 through Nxe2x88x921). The water can be removed from stack 302 by the cathode gas stream. After flowing through the cells, the oxygen flows out of fuel cell stack 302 through outlet 316.
Thus, as the anode and cathode gases are supplied to fuel cell system 300, hydrogen and oxygen are converted into water, and electrons flow through the external load, thereby supplying electrical energy.
The invention relates to fuel cell systems.
Without wishing to be bound to theory, it is believed that fuel cell systems can be susceptible to loss of heat to the environment, particularly at the ends of the fuel cell stack, such as near the end plates. This loss of heat is believed to result in the temperature of the fuel cell stack being non-uniform along the length of the fuel cell stack, and the ends of the fuel cell stack not being maintained at a desired operating temperature of the fuel cell stack. That is, the temperature of the ends of the fuel cell stack, particularly near the end plates, can be relatively lower than the temperature of the center of the fuel cell stack. As a result, during use, as relatively hot reactant(s) containing water passes through the inlets and outlets extending through the service end plate to enter the fuel cell stack, and experiences a temperature drop, water from the reactant(s) can condense in the fuel cell stack. For example, hydrogen coming from a reformer can be relatively hot, e.g., about 65xc2x0 C., and water-saturated, to minimizing drying out of the solid electrolyte in the fuel cell stack. In some embodiments, water may be added to the reactant(s) in a separate step, e.g., pre-humidification. As it enters the fuel cell stack, the hydrogen can experience a temperature drop to, e.g., about 60xc2x0 C., and water can condense in the fuel cell stack.
Condensation of water can be particularly problematic for cell 1. Cell 1 is located at the end of the fuel cell stack where radiative heat loss is relatively high. Cell 1 is also adjacent to an already relatively cool service end plate (due to radiative heat loss). Furthermore, cell 1 has a cathode, which is where heat is generated during operation, that faces away from service end plate 120, vis-à-vis adjacent to the service end plate. Thus, cell 1 is not provided with heating that it may have, for example, if the cathode of cell 1 were adjacent to service end plate 120. Consequently, cell 1 can be relatively cooler, e.g., 2-4xc2x0 C. cooler, than other cells in the fuel cell stack and be particularly susceptible to water condensation.
As water condenses in cell 1, the water can block, for example, the flow channels and the solid electrolyte, and eventually flood cell 1. In a situation sometimes called cell 1 tripping, as cell 1 gets blocked by water and floods, its voltage decreases, and overall performance of the fuel cell stack decreases. Meanwhile, however, cell 2 through cell N may continue to operate normally and to transfer electrons to the cathode of cell 1. As this continues, the polarity of the electrodes of cell 1 reverses from normal operating conditions, e.g., the cathode becomes negative, and/or the potential difference between the anode and the cathode of cell 1 can increase to relatively high oxidizing potentials, such as greater than about 0.6 Volts, or greater than about 1.23 Volts above a Standard Hydrogen Electrode (SHE). At these potentials, the anode of cell 1 can interact with water to produce protons, electrons and oxygen (H2Oxe2x86x922H++2exe2x88x92+xc2xdO2). The protons migrate toward the cathode of cell 1, and the electrons migrate toward the external load, as in the normal fuel cell process.
However, the relatively high oxidizing potential and the evolution of oxygen at the anode of cell 1 can oxidize and degrade materials in the fuel cell, such as, for example, certain catalysts (e.g., ruthenium), catalyst supports (e.g., carbon), and carbon in the gas diffusion layers. These oxidizing conditions can lead to irreversible damage to the electrodes and loss in fuel cell performance. Thus, cell 1 can act as the limiting cell in the fuel cell stack.
Cell 1 tripping can be particularly problematic when the fuel cell system is operating at low power. During operation at low power, gas flow or pressure through the fuel cell stack is relatively slow, and the amount of heat generated by the cathodes in the fuel cell stack is relatively low. As a result, condensation of water can be relatively high. In comparison, when the fuel cell system is operating at high power, gas flow or pressure and the amount of heat generated are relatively high, thereby minimizing condensation of water.
Moreover, condensation of water, which effectively reduces the water content in the humidified reactant(s), and can be comparable to operating the fuel cell system with sub-saturated reactants, can shorten the life of the solid electrolyte by drying out of the solid electrolyte. This can reduce the electrical output and life of the fuel cell stack.
In certain embodiments, the invention features a fuel cell assembly having heatable end(s), e.g., near the end plate(s). The heatable end(s) helps to maintain the temperature of the fuel cell stack at a desired temperature and uniform along the length of the stack. Condensation of water is, therefore, minimized, thereby minimizing cell 1 tripping. The fuel cell assembly can also be operated with saturated reactant(s), thereby minimizing drying out of the solid electrolyte. As a result, the performance and life of the fuel cell assembly can be enhanced.
In some embodiments, the positions of cell 1 and cell N are reversed. That is, cell 1 is positioned adjacent to the blind end plate, and the anode of cell 1 is adjacent to the blind end plate. Cell N is positioned adjacent to the service end plate, and the cathode of cell N is adjacent to the service end plate. In these embodiments, cell 1 adjacent to the blind end may experience tripping as described above, and one or more heatable elements may be used to minimize tripping.
In one aspect, the invention features a fuel cell assembly including a fuel cell stack, a first end plate associated with the fuel cell stack, and a first heatable element adapted to heat the first end plate.
Embodiments may include one or more of the following features.
The first heatable element is different than the first end plate, and the first heatable element disposed between the fuel cell stack and the first end plate. The first heatable element is adapted to be heated electrically. The first heatable element includes a temperature sensor and a resistive thermal device. The first end plate is heatable. The first end plate includes heating elements.
The fuel cell assembly can include a second end plate associated with the fuel cell stack, and a second heatable element adapted to heat the second end plate. The second end plate can be heatable. The second end plate can include a body defining a flow channel.
In another aspect, the invention features a method of operating a fuel cell system having a fuel cell stack and a first end plate associated with a first end of the fuel cell stack. The method includes heating the first end plate.
Embodiments may include one or more of the following features.
Heating the first end plate includes heating a first heating element different than the first end plate. The first heating element is adjacent to the first end plate or disposed between the first end plate and a fuel cell stack. The method further includes flowing a fluid, which can be heated, through a flow channel defined by the first end plate. The method further includes heating, e.g., electrically, the first end plate with a heating element disposed on the first end plate. The method further includes heating a second end plate associated with the fuel cell stack. Heating the second end plate includes heating a second heating element different than the second end plate. The second heating element can be adjacent to the second end plate or between the second end plate and the fuel cell stack.
In another aspect, the invention features a method of operating a fuel cell system having a fuel cell stack and a first end plate associated with the fuel cell stack. The method includes monitoring an operating parameter of the fuel cell system, and adjusting a temperature of the first end plate based on the operating parameter. The method can be performed as a feedback loop.
Adjusting the temperature can include heating a first heatable element and/or flowing a fluid through the first end plate.
The method can include adjusting a temperature of a second end plate associated with the fuel cell stack based on the operating parameter, e.g., power output of the fuel cell system, temperature of the fuel cell stack, or temperature of the first heatable element.
In another aspect, the invention features a fuel cell assembly having a fuel cell stack having a plurality of outer peripheries, e.g., an end or a side, and a heatable element adapted to heat an outer periphery of the fuel cell stack. The heatable element can be adapted to heat a plurality of outer peripheries.
In another aspect, the invention features a heatable end plate for use in compressing a fuel cell stack in a fuel cell assembly in which the heatable end plate includes a body having at least one flow channel.
In another aspect, the invention features a heatable end plate for use in compressing a fuel cell stack in a fuel cell assembly in which the heatable end plate includes a body and at least one of means for inhibiting condensation of water from at least one humidified reactant passable through at least one opening extending through the body, the at least one opening forming a portion of at least one inlet manifold for conducting the at least one humidified reactant to the fuel cell stack, and means for inhibiting heat loss from an end of the fuel cell stack.
In yet another aspect, the invention features a fuel cell assembly having a fuel cell stack, a first heatable end plate attachable to a second end plate for compressing the fuel cell stack therebetween, and wherein the first heatable end plate includes a first body having at least one first flow channel.
In yet another aspect, the invention features a fuel cell assembly having a fuel cell stack, and a first heatable end plate attachable to a second end plate for compressing the fuel cell stack therebetween. The first heatable end plate comprising a first body and at least one of first means for inhibiting condensation of water from at least one humidified reactant passable through at least one opening extending through the body, the at least one opening forming a portion of at least one manifold for conducting the at least one humidified reactant to the fuel cell stack, and first means for inhibiting heat loss from a first end of the fuel cell stack.
In another aspect, the invention features a method for operating a fuel cell assembly. The method includes providing a fuel cell assembly having a fuel cell stack disposed between a first end plate and a second end plate, and heating the first end plate to at least one of inhibit condensation of water from at least one humidified reactant passing through at least one opening extending through the first end plate, the at least one opening forming a portion of at least one inlet manifold for conducting the at least one humidified reactant to the fuel cell stack, and inhibit heat loss from a first end of the fuel cell stack.
Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.